Confine the repositories dropdown to the height of the page (BB-1559)

On an account main page, when overing over the "Repositories" top menu entry, a popup appears with a list of the repositories. If the list is long, longer than the screen, the simple fact of moving the mouse makes the page layout flicker, which is very annoying.

For example on my account page, I have 30 repositories listed, and that doesn't fit in a 1600x1200 screen.

It would be nice to put the list in an iframe or something similar with a limited height, and put a scroll bar in there instead of using the main page scroll bar.